A Detailed Explanation of the Principle of Foliar Fertilizer Absorption by Plant Leaves
The main function of leaves is to carry out photosynthesis using solar energy, while also performing certain biochemical transformations on the synthesized products and transporting them to other organs. Leaves can also continuously exchange substances with the external environment. The form and mechanism of nutrient absorption by the leaf surface are similar to those of roots, and conform to the five basic principles of rational fertilization...
The main function of leaves is to carry out photosynthesis using solar energy, while also performing certain biochemical transformations on the synthesized products and transporting them to other organs. Leaves can also continuously exchange substances with the external environment. The form and mechanism of nutrient absorption by the leaf surface are similar to those of roots, and conform to the five basic principles of rational fertilization, namely the theory of nutrient return, the law of the minimum nutrient, the law of the optimum factor, the law of diminishing returns, and the law of the combined action of factors. However, the structure of leaves differs from that of roots. From the perspective of crop leaf structure, there is a cuticle layer on the surface of the leaf, with a waxy layer outside the cuticle layer; below the cuticle layer are the leaf epidermal cells, and below the epidermal cells are the mesophyll cells. The epidermal cells of the leaf are arranged neatly; those near the upper epidermis are palisade cells, while those near the lower epidermis are spongy cells. On the upper and lower surfaces of the leaf there is also a structure called stomata, which serve as the channel for communication between the interior of the leaf and the outside world. Leaves exchange substances with the external environment through three pathways. The first is through the stomata, which are mainly distributed on the leaf surface; generally, there are more stomata on the underside of the leaf than on the upper surface. The number of stomata on the leaf surface of most annual crops is 100 to 200 per square millimeter. In addition to mainly absorbing CO2 from the atmosphere, these stomata can also absorb gases such as H2O, NH3, NO2, and SO2. The stomata can also release gases such as O2, H2O, NH3, and H2S into the atmosphere. Another pathway for leaves to exchange substances with the external environment is the hydrophilic micropores in the cuticle layer on the leaf surface. The pore size of most of these micropores<1nm，但密度可达1010/cm2数量级。研究表明，角质层由一种带有羟基和羧基的长碳链脂肪酸聚合物组成，这种聚合物的分子间隙及分子上的羟基、羧基亲水基可以让水溶液渗透进入叶内。叶层与外界进行物质交换的第三种途径是通过叶片细胞的质外连丝，像根系表面一样，通过主动吸收把营养物质吸收到叶片内部。因此，叶片与根系一样，对营养物质也有选择吸收的特点。
From this it can be seen that the main pathways for leaves to exchange substances with the external environment are the stomata, the hydrophilic micropores in the cuticle layer on the leaf surface, and the plasmodesmata passing through the leaf cells - these three pathways through which leaves absorb fertilizer.